When platelets are subjected to shear stress, exposed to aggregating agents, or adhere to a substrate, they make available a thromboplastic activity present in the platelets but unavailable under normal conditions. This thromboplastic activity is associated with a large molecular weight lipoprotein, platelet factor 3 (PF3), which has been purified in our laboratory and used to raise monospecific antibodies against it. Using PF3 and anti-PF3 antibodies, experiments have been designed in the proposed study to delineate the role of PF3 in adhesion of platelts to various well characterized artificial surfaces. PF3 availability will be determined quantitatively by immunoradiometric techniques. Kinetics of PF3 availability after adhesion of platelets and how this PF3 availability may influence further adhesion of platelets and adsorption of plasma proteins will be investigated. Effects of certain inhibitors of platelet function on adhesion of platelets and subsequent availability of PF3 by adherent platelets will be examined. In the latter stages of this proposal, extracorporeal devices actually used with patients will be examined to quantitate PF3 availability. It is proposed that, following adhesion to an artificial surface, adherent platelets make PF3 available. The presence of PF3 may promote activation of adsorbed coagulation factors that may attract nearby platelets or may generate traces of thrombin, part of which may remain adsorbed. Adsorbed coagulation factors would then become a focal point for further adhesion of platelets. Thus, PF3 by virtue of its thromboplastic activity may play an important role in blood-material interactions.